1. Technical Field of the Invention
The present invention relates generally to the detection of cracks and more particularly to the nondestructive detection and characterization of cracks.
2. Discussion of the Related Art
The rapid detection and characterization of fatigue cracks are of major importance for many critical structures. Recent developments in the airline industry have initiated a rethinking of conventional nondestructive evaluation (NDE) methods. As the commercial airline fleet continues to age, the NDE community is being challenged to develop rapid nondestructive testing techniques which can ensure the structural integrity of aging aircraft while keeping aircraft down time to a minimum.
Present methods of fatigue crack detection include eddy current, ultrasonic, dye penetrant, magnetic particle, and acoustic emission (AE) testing. Eddy current, ultrasonic, dye penetrant and magnetic particle testing are all localized techniques, that is, only a small area of the structure is examined at a time. In addition, the dye penetrant technique requires a paint free surface. Both ultrasonic and dye penetrant testing depend upon the application of foreign substances to the surface of the specimen which allows for possible contamination. All of the present techniques also depend heavily on the inspector, requiring a skilled operator in order to be effective. Acoustic emission testing has the advantage of being able to monitor large structures, but yields difficult to interpret results. It has been most successfully used in the past to monitor crack growth. See C. B. Scruby, Quantitative Acoustic Emission Techniques, AERE R 11202, July 1984.
U.S. Pat. No. 4,975,855 to Miller et al. discloses comparing the natural frequency responses of a vibrational shaft to an analytical model to determine crack presence and severity, i.e., to perform modal analysis. Particularly, FIG. 9 of this patent shows that as the severity of the crack increases, the natural frequency splits and the difference between the two new frequencies increases. The shaft is vibrated by a directly contacting shaker which induces extraneous frequencies which would complicate any attempt to combine this system with an acoustic emission crack detection system. U.S. Pat. No. 4,188,830 to Mason et al. discloses measuring acoustic emission emitted from opposite crack faces rubbing against each other as the structure is vibrated. Once again, a direct contact vibrator is used, thereby complicating signal analysis by introducing extraneous signals.
In conventional acoustic emission techniques, it is difficult to obtain stress information at the time of emission events under passive loading conditions. Such conventional techniques require a separate sensor technique to determine the stress as a function of time. Although static or linear loading techniques can produce a record of stress versus the acoustic emission event, such techniques have the drawback of requiring further damage to the sample in the form of crack growth or micro-growth as a source of the emission event.